1. Field of the Invention
The present invention relates to a secondary-battery life estimation apparatus for an apparatus provided with a secondary battery, and a life estimation method for the secondary battery.
2. Description of the Background Art
In recent years, a secondary battery has often been used as a power supply system in combination with a solar battery, or a power generator driven by natural energy such as wind power and hydraulic power or artificial mechanical power such as an internal combustion engine. A power supply system combined with such a storage unit stores excess power in the storage unit and supplies electric power from the storage unit to a load apparatus if necessary, thereby utilizing energy more efficiently.
As an example of this system, there is a solar-light power-generation system charging a storage unit with excess power when the amount of power generated by solar light is greater than a power consumption in a load apparatus, and oppositely, when the amount of generated power is smaller than a power consumption in the load apparatus, driving the load apparatus with electric power outputted from the storage unit.
Therefore, the solar-light power-generation system is capable of storing excess power conventionally out of use in the storage unit, thereby enhancing the energy efficiency compared with a conventional power supply system.
In order to store excess power efficiently in the storage unit, the solar-light power-generation system executes control lest the state of charge (below called SOC) of the storage unit should reach 100%, and in order to drive the load apparatus if necessary, it executes control lest the SOC should fall to zero. Specifically, the storage unit is controlled in such a way that the SOC is within a range of 20 to 80%.
This principle is applied to a hybrid electric vehicle (HEV) provided with an engine and a motor. The HEV drives a dynamo and charges a storage unit with excess power if the power outputted from the engine is greater than motive power required for driving, and charges the storage unit by using a motor as a dynamo when braking or slowing.
In addition, great notice has recently been taken of a load-leveling power supply or a plug-in hybrid electric vehicle making effective use of nighttime power.
The load-leveling power supply is a system which stores electric power in a storage unit in the night when the power consumption declines and the power rates lower and consumes the stored power in the daytime when the power consumption peaks. The leveled power consumption leads to constant power generation, thereby enabling power facilities to operate efficiently, or cutting down on an investment in the facilities.
The plug-in hybrid electric vehicle utilizing nighttime power mainly operates EV (electric vehicle) driving which supplies electric power from the storage unit when running at a poor mileage in the streets and operates HEV driving which utilizes an engine and a motor when running over a long distance, thereby reducing the total emission quantity of CO2.
As repeatedly used, a storage device is degraded, thereby lowering the capacity and raising the impedance. If the capacity falls to a reference value or below, of if the impedance rises to a reference value or above, then the storage device needs to be replaced. If the storage device degraded beyond each reference value is continuously used, for example, an emergency back-up power supply can have trouble, thereby requiring a regular knowledge of the capacity or impedance of the storage device in use.
However, it generally takes a long time to measure the capacity of the storage device accurately. For example, a small nickel-hydrogen storage battery is charged for sixteen hours with 0.1 C(mA) after discharged up to a cell terminal voltage of 1.0 V. Then, it is not given a charge or a discharge for an hour, and thereafter, discharged with 0.2 C(mA) up to a cell terminal voltage of 1.0 V. The discharge capacity taken out at this time is a real capacity. Herein, 1 C (=1It) is a current value for nullifying the residual capacity of a secondary battery in an hour when the secondary battery discharges a nominal capacity with a constant current.
Accordingly, measuring a capacity takes a whole day or so, and besides, since a battery cannot be put into operation while the capacity thereof is being measured, preferably, the capacity should not be measured too frequently. Particularly, if a large battery is provided with a dedicated artificial load for discharge, that requires a load for a great amount of electric current, thereby raising costs.
In order to solve the problems, a method has been proposed for estimating the capacity of a secondary battery within a shorter period of time. For example, a method for precisely estimating the capacity of a nickel-hydrogen battery in a short time without measuring a discharge capacity of the battery in practice is offered (e.g., refer to Japanese Patent Laid-Open Publication No. 2005-235420). This says that the battery capacity can be estimated based on a correlation between a variation in voltage and the capacity after a specified time passes from the point of time when a charge is completed for the nickel-hydrogen battery.
Furthermore, a capacity estimation method using an internal impedance for the purpose of estimating the life of a lead storage battery is put to practical use, and thus, a method for predicting a life by extrapolating a temporal variation in estimated capacity has been proposed (e.g., refer to “Tomonobu Tsujikawa, Tamotsu Motozu, Kunio Nakamura: NTT R&D, vol. 50, No. 8, p. 569 (2001)”).
However, in Japanese Patent Laid-Open Publication No. 2005-235420, the battery voltage needs to be measured twice immediately after fully charged and when the specified time passes after fully charged, thereby increasing the number of measurements of the battery voltage.
Particularly, an electric vehicle requiring, for example, a high voltage of 288 to 600 V is provided, for example, with a secondary-battery module formed by connecting in series a plurality of, for example, 240 to 500 cells each having 1.2 V. The secondary-battery module has a high output voltage, thereby making it hard to measure each output voltage of the whole secondary-battery module all at once in a voltage detection circuit which cannot withstand a high voltage. This compels, for example, a plurality of measurements of the terminal voltage of each cell, thereby if each voltage of, for example, 500 cells in series is measured twice, requiring a thousand battery-voltage measurements.
In “Tomonobu Tsujikawa, Tamotsu Motozu, Kunio Nakamura: NTT R&D, vol. 50, No. 8, p. 569 (2001)”, the battery internal resistance needs calculating, thereby complicating the calculation process.